Proofpoint Essentials reports - "550 5.7.1 ...User email address is marked as invalid"

The Proofpoint Essentials MTA's will give a 550 user invalid response to email sent to or from an address that has been marked as Invalid by the SMTP Discovery service.

"550 5.7.1 Recipient {or Sender} address rejected: User email address is marked as invalid"

This often occurs after a discovered address expires without any action having been taken by the admin.

If the email address is in fact desired to be able to send and receive email, it should be removed from the Marked Invalid List. (Users & Groups | SMTP Discovery - Marked Invalid List)

Invalid email addresses are summarized in our SMTP Discovery reports, which have been running weekly since Jan 2011.

See: SMTP Discovery for more information on SMTP Discovery reports

To minimize administration requirements and to ensure an accurate email list for future reference, please consider using Active Directory Sync or Azure Active Directory Sync.

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